Method of making tungsten coils



United States Patent 3,120,054 METHOD OF MAKING TUNGSTEN COILS John ll.Fitzpatrick, Bloomfield, N.J., assignor to Radio Corporation of America,a corporation of Delaware N0 Drawing. Filed Jan. 21, 1960, Ser. No.3,731 7 Claims. (Cl. 29-547) The present invention relates to a methodof making coils of tungsten and particularly concerns a novelcombination of steps for translating an ingot or slug of sinteredtungsten into a relatively closely wound coil of fine tungsten wirewherein the wire is free from objectionable fissures or slivers.

Coils of tungsten wire find wide use in electron tubes. For example,some type of heaters for indirectly heated cathodes, and helices oftraveling wave tubes, comprise coiled structures of relatively finetungsten wire wherein the turns are relatively closely spaced.

Prior methods of fabricating coils of this type have involved firstdie-drawing a swage rod formed from an ingot or slug of sinteredtungsten, to desired wire size, and then coiling the wire so produced toform a coiled structure. The drawing operation usually involves aplurality of drawing steps for reducing the thickness of the swage rodto the desired wire size, which may, for example, be seven mils. Theingot or slug referred to, is initially an elongated structure having arectangular cross-section. Prior to drawing, the initial slug isprocessed in conventional manner to round off its corners to produce aswage rod of circular cross-section for convenient engagement by dieshaving circular openings.

During each drawing step, dies engage the surface first of the swage rodand then of the wire produced therefrom. The engagement is sliding incharacter and is effected with sufficient pressure to producecontraction in the transverse dimensions of the swage rod and wire. Thetungsten crystals in the surface region of the wire engaged by the die,respond in elongation to the aforementioned sliding pressure engagement.Consequently, after repeated drawing steps, the resultant relativelyfine wire will be characterized by a surface including appreciablyelongated tungsten crystals.

So long as the wire is isolated from stress tending to bend theelongated crystals referred to, the crystals will remain in orientationsdetermined by the drawing steps and will contribute to a definition ofthe normal contour of the wire. However, when the wire is bent, as bycoiling, bending stresses are applied to the elongated surface crystals.Such bending stresses result in severe strain at bonded portions of thecrystals, such as end portions of the crystals, thereby urging such endportions outwardly of the normal wire contour. Such strain is relievedwhen one end portion of a crystal subjected to such bending stressbecomes displaced from the normal wire contour. With the strain thusrelieved, the other end portion remains anchored in the wire.

The displacement of a portion of an elongated surface crystal from thenormal wire contour is characterized by several serious disadvantages.The portions so displaced result in the formation of slivers orprojections extending from the normal wire contour and which'in somecases are of such magnitude as to bridge the space between adjacentturns of a subsequently formed coil and thereby produce electricalshorts therebetween. Furthermore, the spaces from which portions of theelongated crystals have been displaced constitute depressions orfissures in the wire which involve appreciable reduction in diameter ofportions of the wire. Where the wire serves as a carrier of electricalpower such reduction in diameter may produce hot spots resulting inrupture of the Wire.

Accordingly, it is an object of the invention to provide an improvedmethod of making a coil of tungsten.

3,120,054 Patented Feb. 4, 1964 Another purpose is to fabricate a coilof relatively fine tungsten wire wherein the turns of the coil arerelatively closely spaced and wherein the wire of the coil issubstantially free from slivers and fissures.

A further aim is to make a closely wound coil of relatively finetungsten wire, having a relatively long life as a conductor ofelectrical power.

In accordance with one way of practicing the method of the invention, arelatively fine tungsten wire, which may be of the order of seven mils,is made by subjecting a swage rod of tungsten to a novel combination ofdrawing, etching and annealing steps, and winding the resultant Wire tocoil shape without the evolution of objectionable slivers and fissuresin the wire. As a consequence, the adjacent turns of the coil may berelatively closely spaced, without danger of electrical shorts betweenthe turns, Furthermore, the absence of fissures in the wire avoids hotspots thereby contributing to long life of the coil as an electricalconductor.

The novel combination of drawing, etching and annealing steps referredto includes a first group of drawing steps to reduce the tungsten swagerod to an intermediate size. The tungsten swage rod to be drawn,constitutes a generally cylindrical body of sintered tungsten powderwhich may have a diameter of about 150 mils. The first group of drawingsteps involves reduction of the swage rod to a wire form of say 17 milsthickness. It will be appreciated that the accomplishment of thisreduction requires an appreciable working of the rod. Such working iseffected by drawing the swage rod axially through a plurality of dies ofsuccessively reduced size, with appreciable force. The force exerted bythe die on the swage rod is both axial and transverse of the rod. As aconsequence, both surface and subsurface crystals in the rod areflattened and elongated axially of the rod. In the example underconsideration, it is estimated that the length of the elongated crystalsis about mils, and their thickness is approximately five microns.

Some of the crystals so elongated are firmly anchored throughout theirlength to the wire body so that they are effectively restrained againstdisplacement from the normal wire contour, even when subjected tofurther drawing and coiling. However, others of the crystals have hadtheir bonds to the wire body weakened, as by a microscopic displacementof a portion thereof from the wire body. Obviously, if the entirecrystal were so displaced no bond at all would exist and the crystalwould drop from the wire body. During subsequent coiling of the wire,the microscopic displacement above referred to of a surface crystal willbe increased appreciably, to produce slivers extending from the wirebody and depressions in the regions vacated by the slivers.

To avoid such formation of slivers and depressions, the novelcombination of steps aforementioned includes an etching step carried outafter the first group of drawing steps described have been performed andprior to coiling the Wire. The etching step comprises immersing the 17mil wire produced by the first group of drawing steps in an electrolyticcaustic bath. The caustic bath may include about 25% sodium hydroxide ina suitable carrier and involving current density of 20 amperes. The wireis permitted to remain in this bath for about from 10 to 15 minutes, toreduce its size to about 15 mils. This etching step serves to remove theloosely bonded crystals from the wire surface by penetrating the regionof partial displacement of a surface crystal as aforementioned, andattacking the loosely bonded crystal from all sides thereof.

The foregoing etching step effects removal of surface crystals from thewire. It does not appreciably afi ect subsurface crystals whioh havealso been elongated by the first group of drawing steps. However, theelongation of such subsurface crystals is not as severe as in the caseof the surface crystals, due to their remoteness from the die and theshielding afforded by the surface crystals. But the subsurface crystalsexposed by the etching step are sufiiciently long, so that furtherelongation thereof during the second group of drawing steps wouldnullify the effects of the etching step in that the newly exposed sur-[face crystals would have the objectionable lengths produced by thefirst group of drawing steps.

The etching step is not relied on as the final step in the method ofmaking a tungsten wire suitable for bending to coil shape withoutincidents of slivers. This is because the etching step producedepressions in the wire surface, as aforementioned, which give rise tohot spots and rupture of the wire when serving as a carrier ofelectrical power. Therefore, the final group of drawing steps isemployed, according to the invention, to flatten out such depressions.

In order that the second or final group of drawing steps maybe performedwithout re-introducing the objectionably long surface crystal formationsremoved by the etching step, the etched wire is subjected to anannealing step prior to the final drawing steps. The annealing step maybe accomplished by passing an electric current of suitable value,through adjacent lengths of the wire in an atmosphere of hydrogen, toheat the wire to a temperature at which the elongated crystals contractin length and form bodies of concentrated mass. Such transformation ofthe crystal shapes, however, adversely affects the ductility of the wireso that it would be incapable of withstanding bending without rupture.

It will be appreciated, therefore, that while the annealing step isadvantageous in reducing the length of the crystals in the wire, it isaccompanied by the disadvantage of reducing the ductility of the wiremate-rial. However, thi disadvantage is overcome by the same meansaccording to the invention, that is relied on for flattening out orfilling the depressions in the wire produced by the etchin stepaforementioned.

This means comprises the final group of drawing steps referred to beforeherein. This final group of drawing steps not only flattens outdepressions produced in the Wire by the etching step, but alsomoderately elongates the aforementioned contracted crystals to impartductility to the finished wire. To avoid excessive elongation of thecrystals during the final drawing step, critical control of such stepsis employed. One way in which such control is manifested is in themagnitude of reduction in wire diameter effected by the final drawingsteps. In the example under consideration, this reduction in diameterwas eight mils, or a reduction of wire size from mils after the etchingstep, to seven mils after the final dralwing step. However, successfulresults have been obtained when the magnitude of final reduction in wiresize falls within a range, 'as pointed out in the following.

It is important in practicing the invention that the etching step becarried out when the size of the wire is from about 100 to about 150%above that of the finished wire. If the wire after etching and annealingis smaller than about twice the size of the finished wire, it limits themagnitude of the final drawing operation to a value that may beinadequate to produce the degree of crystal working required forductility, and to remove the depressions formed by displaced surfacecrystals. If the size of the Wire after etching and annealing is morethan 150% of the finished wire, the final drawing will involve anexcessive working of the surface crystals which may elongate themexcessively and adversely affect their bond to the wire body, therebygiving rise to slivers during a subsequent coiling of the wire.

After the wire is formed to the desired size aforementioned, it iscoiled by suitable means to form a desired coiled structure, such as acoiled heater for an electron tube, or a helix for a traveling wavetube. In neither case are objectionable slivers or fissures in the wireevident in the coiled structures.

It is to be understood that during the several drawing operations, thewire is coated with a suitable lubricant, such as aquadalg.

It will be appreciated from the foregoing that I have provided a noveland useful method of fabricating a tungsten wire according to which twogroups of drawing steps are separated by an etching and annealing step,for simultaneously reducing the wire diameter and controlling theworking thereof to a degree suflicient for satisfactory ductility of thewire required for bending to coil form, while avoiding the formation ofcrystal structures which give rise to slivers and fissures in thefinished coil.

While the method of the invention has been described in association withtungsten, it also has utility in the drawing to wire form of swage rodsmade of metals other than tungsten. Thus, the invention is of advantagein situations when the drawing of a swage rod of any metal, to wire formis accompanied by the formation of surface crystals elongated to adegree to produce slivers and fissures in the finished wire when thelatter is subjected to a forming operation involving the application ofa bending stress to the crystals referred to.

What is claimed is:

1. Method of making a tungsten wire coil comprising drawing a tungstenswage rod having a thickness greater than five times that of thefinished wire to reduce its thickness to a magnitude of from two to twoand one-half times as great as the thickness of the finished wire,whereby elongated surface crystals are formed in the resultantintermediate size Wire, etching said intermediate size Wire to remove atleast portions of loosely bonded ones of said elongated surfacecrystals, annealing said etched intermediate size wire after saidetching step and prior to further working of said intermediate sizewire, then drawing the etched and annealed wire to still further reducethe thickness thereof to the size of said finished wire, and windingsaid finished wire to coil form.

2. Method of making a bent tungsten wire structure comprising the stepsof die-drawing a tungsten swage rod having a size of from about one anda half to about eight times the size of an intermediate wire size toform a wire of said intermediate size, whereby relatively long andloosely bonded surface crystals are formed in said intermediate wire,etching a surface portion of said intermediate wire including saidloosely bonded surface crystals, annealing said etched intermediate wireimmediately after said etching step, further die-drawing said etched andannealed intermediate wire to from two-fifths to onehalf the size ofsaid intermediate Wire, toform relatively short and firm-1y bondedsurface crystals and to reduce the thickness of the intermediate wire tothe size of a desired wire, and bending said desired wire to the form ofsaid structure, whereby said desired wire is free from slivers.

3. Method of reducing the thickness of a tungsten swage rod, having adiameter of about rnils, to the size of a desired wire, comprising firstdie-drawing said rod to effect a predetermined reduction in the sizethereof and to produce a wire of intermediate size of about 17 mils indiameter, ctchingly removing a surface layer of said intermediate wire,annealing the etched intermediate wire prior to further working thereofafter said etching step, and then die-drawing said etched wire to effecta smaller reduction thereof of from two-fifths to one-half the size ofsaid intermediate wire, to produce a wire of final size, whereby saidfinal Wire is substantially free from loosely bonded surface crystalsand the surface of said final wire is free from slivers and fissureswhen bent to desired shape.

4. Method of reducing the size of a tungsten swage rod to that of arelatively fine wire of ductile tungsten and substantially free fromloosely bonded surface crystals tending to produce slivers and fissurestherein when the fine Wire is subjected to bending stresses and whereinsaid swage rod is greater than five times the size of said fine wire,comprising die-drawing raid rod to reduce the size thereof to amagnitude from two to two and one-half times the size of said fine wireand to produce a wire of intermediate size, etchingly removing a surfaceportion of said inter-mediate wire, annealing the etched intermediatewire whereby ductility of said intermediate wire is impaired, andre-drawing said etched and annealed wire to produce said fine wire inductile state, said etching and annealing steps being carried out in[the order named, said intermediate wire being free from any processingstep between said etching and annealing steps.

5. Method of making a tungsten wire coil comprising drawing a tungstenswage rod more than five times as thick as the finished wire to form awire of from two to two and one-half times as thick as the finishedWire, etching the wire so produced to remove a surface layer therefromabout one mil thick and including elongated surface crystals, annealingthe etched wire to contract surface crystals exposed by said etchingstep as the next adjacent step to said etching step in :the processingof said wire, drawing the annealed Wire to the size of said finishedWire, and bending said finished Wire to coil shape.

6. Method of making a closely wound coil of tungsten wire drawn to afinal size, comprising drawing a tungsten swage rod, having a thicknessgreater than one and onehalf times the thickness of a predeterminedthickness, to form an intermediate wire having said predeterminedthickness, whereby said intermediate wire is provided with appreciablyelongated surface crystals and less elongated sub-surface crystals,removing said surface crystfls and exposing said sub-surface crystals,then in an intermediately adjacent step annealing said intermediate wirewith said exposed sub-surface crystals to transform said subsurfacecrystals to bodies of such concentrated mass that further drawing ofsaid annealed intermediate wire to said final size elongates said bodiesto a magnitude less than the length of said appreciably elongatedsurface crystals, said magnitude being suflicient to render said finalsize wire sufilciently ductile to facilitate winding. to said coil formwhile preserving said coil from slivers extending from the normal Wirecontour, drawing said annealed intermediate wire to reduce the thicknessthereof to from two-fifths to one-half of said predetermined thickmessto form said final wire size, and closely winding said final size wireto coil form.

7. Method of making a closely wound coil of tungsten wire drawn to afinished size, comprising drawing a tungsten swage rod having athickness more than five times that of said finished wire, to anintermediate wire size having a thickness of from two to two andone-half times as large as the thickness of said final wire size,whereby surface and sub-surface crystals are elongated in a descendingorder of length in said intermediate Wire, etching said intermediatewire to remove said surface crystals and to expose said subsurfacecrystals, annealing said etched intermediate wire in a step immediatelyfollowing said etching step to transform said exposed subsurfacecrystals to bodies of such concentrated mass that further drawing ofsaid intermediate wire to said finished size only moderately elongatessaid sub-surface crystals to impart desired ductility to said finishedwire and to avoid slivers when said finished wire is wound to producesaid coil, drawing said etched and annealed intermediate wire to saidfinished wire size, and closely winding said final wire to coil shape.

References Cited in the file of this patent UNITED STATES PATENTS1,077,696 Fuller Nov. 4, 1913 1,663,564 Rich Mar. 27, 1928 2,792,627Thall May 21, 1957

1. METHOD OF MAKING A TUNGSTEN WIRE COIL COMPRISING DRAWING A TUNGSTENSWAGE ROD HAVING A THICKNESS GREATER THAN FIVE TIMES THAT OF THEFINISHED WIRE TO REDUCE ITS THICKNESS TO A MAGNITUDE OF FROM TWO TO TWOAND ONE-HALF TIMES AS GREAT AS THE THICKNESS OF THE FINISHED WIRE,WHEREBY ELONGATED SURFACE CRYSTALS ARE FORMED IN THE RESULTANTINTERMEDIATE SIZE WIRE, ETCHING SAID INTERMEDIATE SIZE WIRE TO REMOVE ATLEAST PORTIONS OF LOSELY BONDED ONES OF SAID ELONGATED SURFACE CRYSTALS,ANNEALING SAID ETCHED INTERMEDIATE SIZE WIRE AFTER SAID ETCHING STEP ANDPRIOR TO FURTHER WORKING OF SAID INTERMEDIATE SIZE WIRE, THEN DRAWINGTHE ETCHED AND ANNEALED WIRE TO STILL FURTHER REDUCE THE THICKNESSTHEREOF TO THE SIZE OF SAID FINISHED WIRE, AND WINDING SAID FINISHEDWIRE TO COIL FORM.